Polycarbonate is a thermoplastic that has excellent mechanical properties (e.g., impact resistance), heat resistance and transparency. Polycarbonate is widely used in many engineering applications. It is known that a high level of end-capping, (i.e., wherein most of the terminal phenolic hydroxyl groups in the polycarbonate are reacted with monofunctional endcapping agents to form relatively inert polymer chain ends) helps to reduce static, improve heat aging, and reduce water absorption. Consequently, various coupling agents and end-cappers have been used to enhance the end-cap levels in the production of polycarbonate.
Unexamined Japanese Patent Application H6-157739 discloses the use of certain non-activated carbonates (e.g., diphenyl carbonate) and esters as end-capping agents.
Japanese Patent Application JP-A 7-90074 discloses a method of producing a polycarbonate from a dihydric compound and a carbonic acid diester by an ester exchange method. In this method, a highly active diester, acid halide or acid anhydride with at least two functional groups is added after the ester exchange ratio exceeds 70% to obtain a polycarbonate having an enhanced degree of polymerization. It should be noted that JP-A 7-90074 teaches the use of de-activated molecules as coupling agents or polymerization promoters, and not end-cappers.
U.S. Pat. No. 5,696,222 and European Application No. EP 0 985 696 A1 disclose a method of producing a polycarbonate having a high-end cap levels by adding certain activated and bis-activated carbonates as end-cappers. It is disclosed that the end-capping agents are added to the process after the polycarbonate formed has an intrinsic viscosity of at least 0.3 dl/g. The resulting polycarbonate has increased end-cap levels with minimal changes in molecular weight or intrinsic viscosity (i.e., it has an intrinsic viscosity that is greater or smaller than the viscosity of the polycarbonate formed before the addition of the end-cappers by at most 0.1 dl/g). It is also disclosed that these end-cappers are activated by a phenolic group having an ortho chlorine atom, methoxycarbonyl or ethoxycarbonyl group. These end-cappers are high melting point solids, and thus require complicated and energy intensive apparatus comprising melting vessels and heated feeding lines for accurate and controlled feeding of the end-capper to the polycarbonate.
EP 0 980 861A1 discloses the use of certain salicylic acid ester derivatives as terminal blocking agents in amounts of 0.1 to 10 times, and most preferably 0.5 to 2 times, mole per mole equivalent of terminal hydroxyl groups of the polycarbonate formed at a time of the addition. Such polycarbonates have good color tone suitable for optical material use. It is disclosed that these end-cappers are activated by a phenolic group having an ortho methoxycarbonyl or ethoxycarbonyl group. It should be noted that the Examples of EP 0 980 861A1 teach the use of 2-methoxycarbonylphenyl-phenylcarbonate as an end-capper in an amount that is about 1 mole per mole equivalent of terminal hydroxyl groups to form a polycarbonate with increased end-cap levels.
There is still a need for an improved melt process using easy to handle low melting end-cappers to produce polycarbonate having capped terminals and controlled molecular weight.